1. Field of the Invention
The present invention relates generally to an evaporation control system for an internal combustion engine and particularly to a fuel vapor purging control system which is capable of controlling a purge rate of fuel vapors from a fuel vapor storage canister over a wide range of engine operation.
2. Description of the Prior Art
Japanese Patent First Publication No. 59-192858 discloses a canister purge control system for an automotive vehicle. This system includes a variable displacement pump arranged between a canister and an induction passage of the engine. The variable displacement pump is controlled according to engine operating conditions over a range of low to high engine loads to draw a purging stream of air through the canister so as to purge collected fuel vapors from the canister. Therefore, even when the engine is operating under high load conditions, a desired purge flow rate may be assured.
The above prior art canister purge control system, however, encounters a drawback in that the variable displacement pump is always driven by the engine even during low engine load operation where the engine manifold vacuum level is sufficient for drawing ambient air into the canister for purging fuel vapors collected therein, thus resulting in engine load being increased undesirably.
Additionally, Japanese Utility Model First Publication No. 61-17466 discloses a canister purge control system which includes two purge lines each communicating between a canister and an induction passage of the engine. A purge control valve is arranged in one of the purge lines, which is energized to modify a purge flow rate through the canister when engine load is relatively low. An air pump is disposed in the other of the purge lines, which serves to draw air and fuel vapors collected in the canister into the engine for engine starting control. With this arrangement, when the engine load becomes high, the air pump may also be utilized to purge fuel vapors from the canister. However, assuring a desired purge flow rate at high engine loads requires finely controlling the air pump.